Envisions Mathematical Expression Language - An Overview
ENVISION includes a mathematical expression language that can be used to evaluate map-based mathematical expressions. This is used in a variety of “standard” plug-ins, and can be accessed using user-defined plug-ins. This language allows for expressions that can contain:
- Algebraic operators (e.g. add, subtract, etc.)
- Logical operators (e.g. logical AND, logical OR, etc. )
- A variety of mathematical functions (e.g. sin, cos, etc.)
- Field references for accessing IDU-level field information (e.g. AREA, IDU_INDEX, POPDENS)
- Expression precedence grouping using parentheses.
Available operators and functions are specified below.
Operators
| Operator | Description | Example |
|---|
| + | Addition | 1 + 2 |
| * | Subtraction and unary minus | 1-2 |
| * | Multiplication | 1*2 |
| / | Division | 1/2 |
| ^ | Power | 2^2 |
| % | Modulo | 2%1 |
| & | Logical AND | MYFIELD & 2 |
| | | Logical OR | MYFIELD | 2 |
| ! | Logical NOT | ! MYFIELD |
| >, >= | Greater or equal to | MYFIELD > 3 |
| <, <= | Less than or equal to | MYFIELD <= 3 |
| != | Not equal | MYFIELD != 3 |
| == | Equal | MYFIELD == 3 |
Built-in Functions
| Operator | Return Value | Example |
|---|
| abs(x) | Absolute Value of x | abs(-12) => 12 |
| acos(x) | Inverse Cosine of x (radians) | |
| asin(x) | Inverse Sine of x (radians) | |
| atan(x) | Inverse Tan of x (radians) | |
| avg(x,y,z...) | Average value of the arguments | avg(1,3) => 2 |
| ceil(x) | Value of x rounded up to nearest integer | ceil(2.3) => 3 |
| cos(x) | Cosine of x (radians) | |
| cosh(x) | Hyperbolic cosign of x (radians) | |
| floor(x) | Value of x rounded down to nearest integer | floor(2.3) => 3 |
| if(cond,trueEx, falseEx) | Conditional expression - if 'cond' is true, return value of 'trueEx', otherwise, return the value of 'falseEx' | |
| log(x) | Natural Log of x | |
| log10(x) | Log base 10 of x | |
| max(x,y,z,…) | Maximum value of the argument list | |
| min(x,y,z,…) | Minimum value of the argument list | |
| rand(x) | Uniform Random value between (0,1) | |
| rand(min, max) | Uniform Random value between (min,max) | |
| round(x) | Value of x rounded to the nearest integer | |
| sin(x) | Sine of x (radians) | |
| sinh(x) | Hyperbolic Sine of x (radians) | |
| sqrt(x) | Square Root of x | |
| sum(x,y,z,…) | Sum of the argument list | |
| tan(x) | Tangent of x (radians) | |
| tanh(x) | Hyperbolic Tangent of x (radians) | |
Compound Expressions
Expression can be nested to an unlimited depth. Examples of valid expressions include:
- 1 + sin( 0.5 )
- 17 * AREA + log( POPDENS ) - Note: this assume AREA and POPDENS are fields in the IDU database
- if ( area > 100, AREA * 1000, 0 ) - Note: syntax is if ( condition, expr when true, expr when false)
For more details on the expression evaluator used in Envision, see http://www.codeproject.com/Articles/7335/An-extensible-math-expression-parser-with-plug-ins